The info presented here
applies to unmodified Volvo (*)
544, 122 and 1800 (Bosch generator) 12V electrical systems, and is intended to
help in the understanding of these systems for the purpose of trouble locating
and repair after the AMP indicator comes ON. I have long ago upgraded just
about all of my vintage vehicles to modern, high output alternators with fully
electronic internal regulators, and I’m a big advocate of this, but I thought I
would compile this article for those who prefer to remain original, as locating
the troubles once the AMP indicator comes ON can be tricky for qualified
individuals and totally baffling for the inexperienced. The article is not
yet complete and neat, but there is enough info here to get one well started...and
now that I've added a detailed FIGURE 1 plus explanation, it is really getting
in need of a rewrite/cleanup...

Notice:
I stand behind all the information presented here, having put it together with
the utmost care, but in the end, the reader is responsible for its use and the
consequences of their own actions. The information presented should be used in
conjunction with normal, cautious shop practice. I will not be responsible for
your actions and knuckles. Work safely!

----------------------------------------

Since the deceivingly simple
AMP indicator is actually employed in a slightly complicated manner (not to
mention that is not even indicating AMPs - that was just a handy - and short,
electrical sounding word that would fit on the tiny indicator), a complete
understanding of this indicator’s function is necessary. It may, in some
circles, be called an “Idiot Light”, but the manner in which it was designed
into the electrical system is anything but idiotic…its actually rather
ingenious…in a do-the-maximum-with-a-minimum kind-of way!

The AMP light itself is a
dirt-simple (3 Watt) incandescent lamp, but the manner in which is cleverly
worked into the electrical system allows it to light up and indicate two very
different failure conditions. Herein lies the complication. In the first
place, in the normal condition (IGNition ON, engine running / all-things-fine),
the lamp actually has voltage applied to both sides, and so does not
light! Understanding this simple fact is essential to appreciating what is
happening here, and unraveling the complication. Reminder: For any electrical
device to function it requires voltage across its terminals. This
voltage serves to encourage the flow of electrical current through the
load, powering it up…and indeed, the AMP indicator as well, must have voltage
across it to light!

Reviewing, a complete
122S wiring
diagram, or FIGURE 1. below, it can be seen that, one side of the AMP indicator is
connected to Fuse 1 (IGNition power). The other side is connected to, and
monitors the charging system output at, the
Voltage Regulator (VR) D+ terminal. The reader will note and should
understand, that in this situation, with voltage on both sides of the lamp, no
current flows, and the indicator remains unlit!

Again, in order to light the
lamp, the voltage-across condition must be satisfied. Only this will
allow current to flow. The really observant or electrically enlightened reader
will also notice, that this condition may be met by tying either side to
ground. [See:
Idea 1 below]

Condition 1: Normal, non-failure mode.

The two conditions, which
will cause the AMP indicator to light while the engine is running, are
therefore:

Condition 2:
Failure of charging system (…the armature of generator does not produce voltage,
but instead, completes circuit to chassis allowing lamp current to flow…for
example…when IGN key is turned ON, but before starting the engine).

Condition 3:
Fuse 1 open (…at which time, the other Fuse 1 loads complete the circuit to
chassis allowing lamp current to flow).

The “Other” Condition: AMP indicator glows slightly while
engine is running. This is a frequently occurring condition known to just about
all owners of these vehicles, discussed separately below.

----------------------------------------------------------

Condition 1is the normal non-failure condition of Ignition ON and Charging system
output normal.

Condition 3
Checkout of open Fuse 1 causing AMP indicator to be ON.

An open Fuse 1 is quickly
verified, as it is accompanied by the loss of function of other Fuse 1
loads…specifically, these are: Directional indicators, Fuel gauge, Blower, and
Wipers. If the AMP light ever comes on full, while the engine is running, or
does not extinguish shortly after starting, this is the first thing, which
should be checked for, as it is the simplest. The driver, even a technical
layman can, and should do this…as soon as practical.

Functional test when AMP
indicator comes ON with engine running:
Turn any of the other Fuse 1 loads ON. If loads do not function, Fuse 1
should be suspect. Open hood at the next opportunity, and with IGN ON and
engine running, activate left directional indicators (these are easily seen
while standing next to the vehicle on the drivers side). Remove the fuseblock
cover (you should know where it is!),and visually check the state
of Fuse 1. If the fuse is visually OK (Suggestion: Always install Euro-fuses
with element visible to allow for an easy inspection!), spin it in its holder…if
this restores the connections so that the directional indicators start blinking
(and causes the AMP light to extinguish), corrosion may have been present and
breaking the connection. Spinning the fuse has cut through this corrosion at
the conical fuse ends, and reestablished a connection… This is actually a
rather common failure…on the injected 140s and 1800s, this failure occurring on
the fuel-pump relay fuse is also well-known…

…if, on the other hand, the
wiper worked when the switch was tried,
Condition 2 may exist, or the classic
“Other” Conditioncaused by
long-term corrosion. Checkout and correction of
Condition 2will require a more careful investigation of voltages
under the hood using a multimeter. This is down-and-dirty technical stuff, and
should be performed by someone who is at least familiar with the
operation of a multimeter.

Verification of open Fuse 1
(or associated connections) condition:
Shut IGNition OFF, allowing motor to come to a stop. Turn IGN to the ON
position, but do not START. If AMP indicator does not light, Fuse 1 should be
suspect. Explanation: When IGN power is applied, but before starting, the AMP
indicator is supplied on one side with IGN power and on the other with a
connection to chassis through the non-electric producing armature of the
generator. The AMP indicator should light…if it does not, it must be assumed
that it is not being supplied with power, and therefore Fuse 1, or associated
connections are suspect.

If replacing a (blown) Fuse
1 remedies the condition …it’s a mixed blessing. This is the simplest failure
to fix, but one has to wonder why the fuse actually blew in the first
place…because actually blowing the 25A rated Fuse 1 is fairly unusual.

If on the other hand, just
the action of moving the fuse in its holder (or spinning it) is enough to
restore function, that suggests the more common poor connection due to corrosion
condition.

Consider yourself lucky for
not having a dreaded charging system failure , but deduct two points for neglect, and,
at earliest possible opportunity, clean fuse-ends and the circular
connection holes in the clips properly with abrasive, apply Zinc Anti-Corrosive Electrical Paste,
and reinstall fuse. Suggested reading:
Gas-Tight-Joint Tech article.

Having checked Fuse 1 and
found it to be intact, and spinning it to try to cut through any corrosion did
not fix the symptoms, another check, requiring a multimeter should be performed
before moving on to checks of the charging system. With the IGN ON and engine
running, the IGN power should be measured at the Fuse 1 and Fuse 2 (input)
terminals on left (Black wires), as well as (output) terminals
(Red wires), on right side. If voltage is measured at the input but not the
output, an open connection at the fuseblock exists…this may be caused by a poor
connection due to corrosion at the push-on terminals, corrosion between the
riveted contact plates or conical fuse-ends (as both discussed in
the Gas-Tight-Joint Tech article), or a blown
or fatigued fuse which was previously missed.

If checks around the
fuseblock reveal no problems areas, a failed charging system must be considered
to be causing the AMP light to be ON..

Condition 2
Checkout of Charging System causing AMP light to be ON. This is the next thing,
which should be checked. It is down and dirty technical stuff, requiring tools,
a multimeter and at least a basic understanding of how to correctly put these to
use without hurting oneself (knuckles not withstanding).

With engine running, measure
voltage from D+ terminal (Generator output) of VR to chassis. If this voltage
is less than 2 Volts, charging system (combo of the generator and regulator) has
no output.

Regulator Bypass Test:
[As the generator is not regulated during this test it should only be performed
momentarily...that is, just long enough to perform the test]. While continuing
to measure the output voltage at D+, short the DF terminal of the VR to chassis
(all wiring normally in place), using a screwdriver or similar at the regulator
DF terminal. This takes the place of the VR and completes the path to ground,
allowing field current to flow (see:
Simplified Charging System Wiring
below). An output in excess of 12V should now be read on the D+ terminal, and
this voltage should also be measured, without a drop, on the battery plus
terminal. Raising the RPMs should raise this voltage, as the generator output
comes up. This suggests a Voltage Regulator problem. Replace VR. Note:
Successful repair of the VR is beyond the scope of this article and without
calibrated test equipment and setup, likely beyond the abilities of even the
most experienced backyard wrencher! There is a lot happening in the
Voltage Regulator! (Link: Bosch Generator based Charging
System in Detail)

If bypassing the VR does
not result in an output, the generator itself must be suspect.

> Are you certain the Generator is the problem?
>
> If you are certain the problem lies with the Generator and have spares and or
a competent shop which can repair/rebuild it, or can do it yourself, that
certainly is one alternative (check sw-em site for hints and troubleshooting
info) but you or the shop you entrust the project to would have to be one
familiar with the Bosch Generator based charging system, or the project has only
a small chance of having a successful outcome.
>
> Often however, if the problem lies with the Charging Control...better known as
the Voltage Regulator, which has mechanical contacts and which is therefore
subject to wear...repair could be even trickier...(Link: Bosch Generator based Charging
System in Detail)
>
> The Alt upgrade on the other hand has a lot going for it, which you are
undoubtedly aware of...and reliability is definitely one of them...

---

A simple test to bypass the VReg and manually call for Generator
output while a Voltmeter is connected to battery. Raise engine RPMs to
about 2K, then for purposes of test, use a test wire to connect Df terminal of
VReg to chassis (this bypasses VReg and applies Field current manually, by
completing connection to chassis, which Vreg would normally)... monitor system
voltage. If it noticeably rises, and system/battery voltage starts climbing,
indicating Battery is charging, it suggests Gen is OK and putting out, and VReg
is not working as expected. Testing VReg is beyond the casual mechanic as
it requires special equipment, techniques and experience.

Of course all terminals and connections involved should be clean and
tight before undertaking any of this...I highly recommend loosening,
cleaning with emery cloth, and reconnecting with a film of ACZP (send me
a SASE, and I'll send a freebie sample!), and tightening all electrical
connections having anything to do with voltage monitoring and charging
system....those are: All VReg (including chassis mounting),
Generator, Battery, Ignition Switch, Engine to Chassis braid, Fuse 1
connections. (including fuse-ends and FB terminals).

------------------------------

Generator checks.

In-Situ Reciprocity Test.
This is a test, helpful for determining the state of health of the generator.
It is performed which the generator installed and wired normally, but
with the fanbelt removed. A side-effect here is that the wiring from the VR is
checked at the same time. A successful test, gives a certain level of
confidence in the generator, harness and connections. Performing the
Reciprocity Test is simple…after loosening and removing the fanbelt, use two
temporary test jumper wires to 1. Connect the DF terminal to chassis, and 2.
Connect the B+ to the D+ terminals at the VR. This applies battery power and
field current to the generator, and by applying a marvelous principle of
science, which states that many processes run backwards as well as forwards, the
generator should spin as a motor. If it does not, Generator is suspect. Remove
test wires and proceed to…

Passive Resistance Tests.
With IGNition OFF, battery plus disconnected, and loose fanbelt, disconnect the
D+ and DF wires from the generator. With the multimeter in the Ohms function,
verify the internal connections as shown in the graphic following. The D+
terminal (Armature output, which runs through the brushes) to chassis reading
should read about:: 50 Ohms, remaining fairly steady and without a lot of
fluctuation when spinning the generator pulley in the normal running direction.
The DF to D+ terminal should read about 4.8Ohms, unaffected by a spinning
armature. This is a basic check on internals of the generator as shown
below. Note that the power connection for the field is permanently made,
within to the generator, but requires an external connection to chassis in order
to allow field current to flow. This in fact is, what the VR does as it
monitors the system voltage against an internal reference, and what the
troubleshooter has done, manually, when performing the previous
Regulator Bypass Test.

D+ DF
Source: Volvo Factory Service Manual

Poor Connections!
Don’t discount these, since poor connections are a definite possible cause of
electrical problems, especially on a 40 year old vehicle, it wouldn’t be such a
bad idea to assure all connections are clean and tight before starting to
replace expensive electrical components. One would feel pretty silly if hours
of troubleshooting and expensive replacement work were all for naught, if a
problem turns out to really have been caused by a corroded or loose connection!

Zinc anti-corrosive electrical paste
is the Anti-Seize of the electrical world! Use
this paste after disconnecting and cleaning electrical connections of the
vehicle. The connections critical to the charging system are, voltage regulator
(including its mounting bolts which make the connection to the vehicle
chassis!), generator, battery, starter, IGN switch, fuseblock, groundstrap
(connecting chassis and engine).

The “Other” Condition:
AMP indicator glows slightly. The well-known condition of a dimly lit AMP light
is not a separate condition, but a variation of
Condition 1. It is caused by the slight voltage drop, which occurs,
by design, across the Fuse 1**, as the indicator lamp is a small, sensitive
lamp, even a slight voltage across it will satisfy the voltage across
rule, and cause a slight glow of the lamp. This is absolutely normal,
and even occurred when the car was new! If however, additional voltage drop
occurs due to poor connections, the voltage across the lamp increases, and the
lamp glows more brightly.

The effect of a dimly lit
AMP Indicator (particularly evident when turning ON Fan Switch* in dark
conditions) is a direct result of voltage drops due to current passed (IR Drop
**). Specifically, as can be seen in FIGURE 1. , Voltage Drop across
Fuse1 (which is inherent for any fuse and cannot be avoided) plus associated
connections in Fuseblock (which can and should be avoided), is caused by passing
the currents of the downstream loads.

* The observant reader may
have noticed that the dimly glowing AMP Indicator effect is definately more
pronounced when turning ON Fan Switch than when turning ON Wipers, although both
are Fuse1 loads and the Wiper current is higher in magnitude. This seemingly
inconsistent action can be explained by the fact that the Voltage Drop resulting
from Fan current is actual higher than the Voltage Drop resulting from Wiper
Motor, and this occurs because of the wire gauges involved…a heavy wire gauge
(4mm2) runs between Fuse1 and Wiper Switch and a much lighter gauge
wire (1.5mm2) connects Wiper Switch to the Fuel Gauge...and since
voltage drop on a wire for a given current is less on a heavy wire than a thin
wire, it should be clear why the AMP Indicator typically gets brighter when
setting the Fan Switch to the High position while being largely unaffected by
the Wiper Switch

** IR Drop Explained: For the non-engineer reader, the IR Drop is simply
the Voltage which develops across any resistance in the path of current. The
term comes from Ohm’s Law. Normally, the IR Drop is ignored and not spoken of
because components and conductors in a circuit are intentionally sized when
designing the circuit, so that these Voltages are miniscule, because any voltage
dropped along the way is not available at the load to drive current through it
and do the intended work. In other words, when things were new, things were
simple: Voltage at the Source was controlled by some sort of Switch and it was
either ALL across the Switch (when Switch is open) OR ALL across the Load (when
Switch is closed). In the vintage vehicle world however, corrosion and loose or
otherwise poor connections can, and often do, become significant contributors to
circuit action. This must be carefully considered and taken into account when
troubleshooting issues that arise!

-----------------------------------------------------

Summary

In order to light the AMP
indicator, the voltage-across-condition must be satisfied. The two conditions
are:

1.Fuse 1 open

2.Failure of charging system.

Given good connections,
there are two conditions under which the AMP indicator may light, but add to
that, frequently occurring faults due to subtle corrosion and/or poor
connections, locating and remedying the trouble can get complicated…fast!

Following an order of
Simplest-to-Complicated fault isolation, check Fuse 1 and associated connections
first, then Charging system, breaking this into Generator, then Voltage
Regulator, components.

----------------------------------------------------------

Voltage
Regulator (VR) function for a generator electrical system explained:

1. The VR’s main function
is to control the electrical system by monitoring system voltage and activate
the generator to contribute electrical power when system voltage (and therefore
battery charge) is low.

3.A second function is required in generator systems. That is, to
disconnect the generator from the battery when the IGN is OFF and generator is
not contributing (in fact, the generator would take (and waste) battery power if
left connected under this condition).

A fuse is an intentional
weak electrical link, which has a small but important resistance (R - Ohms) to
the flow of electrical current (I - Amps). As current flows to the downstream
load, this resistance causes a small voltage drop (due to Ohms Law V=IR), which in turn causes power
dissipation (P - Watts) to occur (also known as I2R heating). As the current
increases, so does this heating (exponentially, in fact). By designing and
constructing fuses carefully, these can be used as simple, effective, circuit
protective devices, with a predictable (normal and expected) level of current
which will be allowed to pass, and a predictable amount of (fault) current which
will, stop the fault current from being allowed to pass. The fuse “blows” (melts, breaking the connection
and thereby power delivery to the load), when excessive current is drawn by the
circuit it is in series with.

Idea 1.
It is easy to see that depending on which side is supplying the power and which
side is supplying the ground (return current path to battery negative),
determines the direction of current flow through the indicator. This
immediately brings to mind those cute little bi-color LEDs, which have two
different color LEDs in one package, back to back, and so light up differently
depending on current direction. These might be nice to use here…Green for Fuse
1 blown, Red for Charging system failure.

From that thread, Derek was kind enough to post this picture with measured
dimension,
of a new Volvo PN 273500 Generator brush.

One can see on the picture that once Brush is worn past about the 50% point,
its' width will be more than its' height. This is what Leif Anderson's
"road-fix" below is based on...

Excerpt from a post of mine in that thread:

"I specifically recall years ago...Leif Anderson, while visiting the US and
joining us for the Mt. Snow Volvo Meet...when someone's Ch Sys failed due to
short/worn Brushes, he nonchalantly walked by a bunch of guys up to their elbows
in underhood Volvo dirt and Generator dust in the parking lot of an event (when
we should have been hobnobbing and drinking Chablis with everyone else) and
suggested simply putting the same Brushes back into guide with a 90deg twist, as
they were worn shorter than in the wider dimension...this made them taller and
able to get more preload force from the Spring...and it worked!...to return
function of Ch Sys as a temporary fix...long-term, this leaves a step-wear in
commutator so requires proper service including removing any step from
commutator ASAP, but is a good emergency road fix in a pinch."

...now that's what I call thinking "outside the herring tin" (Swedish equivalent
of a popular saying)!

Additional:
The official green Volvo Service manual calls for a nominal field coil
resistance of 4.8 Ohms. No armature readings are given. After a call on the
Brickboard and
Volvoniacs fora, for actual resistance readings on generators made, some
helpful owners (Thanks Phil, Joe, McMike ) provided readings on known good
units, and also suspected bad units. The typical good readings are included in
the previous text for reference. The readings from suspected bad units are
included below along with comments to help owners troubleshoot these
generators. Finally, service recommendations are made for repairing the
non-working units.

-----------------------

Suspected Unit 1

D+ to D- (Armature,
static): Open (infinite Ohms)

D+ to D- (Armature, while
turning): Open

DF to D+ (Field only): Open

Comments:
When making resistance measurements on old, possibly less than clean automotive
components, its obvious to assure good connections are made…so the first thing I
would ask in response to these readings is: Are your sure you were making a
good connection with the meter probes? If the answer is: “Yes”, the second
thing I’d ask is: Are there any guts in the thing or is it perhaps just an
empty shell?

Service Recommendation:
Open inspection band, retake readings on wires or connections inside to verify,
if no change, disassemble totally, and recheck Armature and Field resistances
while apart.

-----------------------

Suspected Unit 2

D+ to D- (Armature,
static): 48Ohms

D+ to D- (Armature, while
turning): Bouncing from 20 to 200 Ohms

DF to D+ (Field only): 4.8
Ohms

Comments:
Possibly worn brushes, possibly a contaminated commutator, or both. Field
resistance is nominal, so this generator has possibilities!

Service Recommendation:
Open inspection band, check height of brushes and color of commutator, using
emery board clean commutator to a shiny copper, clean using carb cleaner or
brake cleaner solvent to flush (conductive) dust from brushes, and copper
particles from commutator clean-up, then compressed air to dry.

> Are you certain the Generator is the problem?
>
> If you are certain the problem lies with the Generator and have spares and or
a competent shop which can repair/rebuild it, or can do it yourself, that
certainly is one alternative (check sw-em site for hints and troubleshooting
info) but you or the shop you entrust the project to would have to be one
familiar with the Bosch Generator based charging system, or the project has only
a small chance of having a successful outcome.
>
> Often however, if the problem lies with the Charging Control...better known as
the Voltage Regulator, which has mechanical contacts and which is therefore
subject to wear...repair could be even trickier...
>
> The Alt upgrade on the other hand has a lot going for it, which you are
undoubtedly aware of...and reliability is definitely one of them...

---

A simple test to bypass the VReg and manually call for Generator
output while a Voltmeter is connected to battery. Raise engine RPMs to
about 2K, then for purposes of test, use a test wire to connect Df terminal of
VReg to chassis (this bypasses VReg and applies Field current manually, by
completing connection to chassis, which Vreg would normally)... monitor system
voltage. If it noticeably rises, and system/battery voltage starts climbing,
indicating Battery is charging, it suggests Gen is OK and putting out, and VReg
is not working as expected. Testing VReg is beyond the casual mechanic as
it requires special equipment, techniques and experience.

Of course all terminals and connections involved should be clean and
tight before undertaking any of this...I highly recommend loosening,
cleaning with emery cloth, and reconnecting with a film of ACZP (send me
a SASE, and I'll send a freebie sample!), and tightening all electrical
connections having anything to do with voltage monitoring and charging
system....those are: All VReg (including chassis mounting),
Generator, Battery, Ignition Switch, Engine to Chassis braid, Fuse 1
connections. (including fuse-ends and FB terminals).

You are welcome to use the
information here in good health, and for your own non-commercial purposes, but
if you reprint or otherwise republish this article, you must give credit to the
author or link back to the SwEm site as the source. If you don’t, you’re just a
lazy, scum sucking plagiarist, and the Boston Globe wants you! As always, if
you can supply corrections, or additional objective information or experience, I
will always consider it, and consider working it into the next revision of this
article...along with likely the odd metaphor and probably wise-a** comment.